This invention relates to the general field of gas turbine engines and more particularly, to a method of achieving light-off fuel flow under a variety of altitude and temperature conditions.
In a gas turbine engine, inlet air is continuously compressed, mixed with fuel in an inflammable proportion, and then exposed to an ignition source to ignite the mixture which will then continue to burn in order to generate the combustion product.
The starting of a gas turbine engine is a complex process and generally includes two stages. In the first stage, the gas turbine engine is rotated by a torque provided by an external source, for example, by a starter. When a predetermined compressor pressure or speed is reached, fuel flow is injected at a controlled rate into the combustor to mix with the air flow and the mixture is exposed to an ignition source and eventually ignition occurs. In the second stage the fuel flow is continuously injected into the combustor, enabling the local ignition to propagate and spread in order to form stable combustion in the combustor. During the second stage, the engine speed is accelerated by increasing the fuel flow injection until the engine operates under a self-sustained speed.
As part of engine design testing procedures, gas turbine engines must be able to start under conditions involving a range of temperatures and altitudes. Altitudes can vary from as low as a few thousand feet below sea level to altitudes that are greater than 65,000 ft. above sea level. Temperatures can range from xe2x88x9260 degrees F. to +135 degrees F.
The typical engine fuel control will provide a fixed starting fuel flow for light-off based on the ambient pressure and ambient temperature. In the case of in-flight re-lights, the effects of ram may also be input into the fuel control system in order to further bias the fuel flow for light-off.
The engine""s requirements for light-off fuel delivery vary significantly with engine size, the number of nozzles, the type of nozzles used, the altitude, the temperature of the air and fuel, the viscous and aerodynamic drag effects on the rotors and the forward velocity of the engine. Increasing altitude causes a rarefying of the air and a need to reduce the light-off fuel flow. Very cold temperatures cause a need for higher fuel flows in order to achieve light-off. In particular, at very cold, high altitude static starts, a high light-off fuel flow requirement may be needed in order to initiate light-off. However, sustaining this flow may result in overtemperatures in the turbine area and the associated stresses that follow. In addition, this high fuel flow required for light-off may lead to visual flame being emitted from the jet pipe or exhaust of the engine. Longer light-off times may result in fuel pooling. Once this pooled fuel finally burns, visible torching may result. This torching is highly undesirable as it may also lead to engine distress on the turbine blading.
Efforts have been made in the industry to improve gas turbine engine starting, particularly for reducing light-off time which is taken from the point of fuel injection to the light-off occurrence, in order to have a quick start-up of the engine. U.S. Pat. No. 5,718,111, issued to Ling et al. on Feb. 17, 1998, as an example of such efforts, describes a gas turbine engine start-up control system and method in which the engine exit temperature and the compressor speed change rate are sensed, and the sensed parameters are compared with desired start-up characteristics and referenced to look-up tables for determining an output composition factor. Based on the output composition factor, the start-up of the gas turbine engine is adjusted, generally by adjusting fuel flow through use of a fuel control system. However, this method and system are generally used for controlling fuel flow in the second stage of engine start-up because the engine exit temperature changes after the light-off occurrence.
U.S. Pat. No. 6,062,016, issued to Edelman on May 16, 2000 discloses a gas turbine engine light-off system and method in which the gas turbine engine is operated at a fixed speed in order to provide a substantially constant supply of combustion air for light-off, and the fuel flow is ramped up to achieve the correct fuel-to-air ratio for light-off.
U.S. Pat. No. 5,107,674, issued to Wibbelsman et al. on Apr. 28, 1992 describes a starting system for a gas turbine aircraft engine. The starting system automatically controls the sequencing of events needed during engine start-up that lead up to light-off, including sensors for ambient temperature, exhaust gas temperature, compressor speed, fuel flow, etc. The control schedules fuel flow in a manner which avoids stalls and takes corrective action when stalls occur, and provides scheduling of fuel flow in severely cold conditions.
U.S. Pat. No. 5,369,948, issued to Vertens et al. on Dec. 6, 1994 describes a process and apparatus for starting a gas turbine engine. In this apparatus with a start-up controller for a gas turbine engine, the amount of fuel injected by controlled dosing pumps is determined, whereby the amount of fuel injected can be regulated as a function of the difference between the injection pressure and the compressor pressure in the combustion chamber of the turbine, or at the compressor outlet.
Nevertheless, there is still a need for a better engine starting method for reducing light-off time of engine start-up and for avoiding injection of excess fuel during engine start-up, particularly under cold weather conditions because the fuel accuracy of achieving light-off of gas turbine engines becomes more critical as the fuel and air temperature get colder.
A method of engine starting in a gas turbine engine according to one aspect of the present invention comprises steps of rotating the engine to provide an air flow into a combustor of the engine, and injecting fuel into the combustor at a varying rate until the engine is lighted-off. The varying rate of fuel injection is a function of time and is represented by a curve having at least one high frequency which represents instant changes of the rate for intersecting a light-off zone. The curve preferably further has a low frequency with respect to the light-off time, representing a change trend of the varying rate. After the light-off occurrence the engine is accelerated to a self sustaining operation condition by continuously injecting fuel into the combustor. The engine may be rotated at a varying speed as a function of time, such as an increasing speed, or rotated at a fixed speed, in order to achieve light-off. The curve for intersecting a light-off zone preferably has an increasing trend and comprises, for example, an oscillatory profile, a squared-off wave profile, a step profile or a series of spikes, according to various embodiments.
The mixture of fuel and air can be ignited, which is generally referred to as light-off, in the combustor of a gas turbine engine with a fuel/air ratio falling within an appropriate range. This fuel/air ratio range for light-off is affected by fuel viscosity, atmospheric temperature, and atmospheric air pressure. The air pressure is primarily determined by the compressor speed and the altitude where the gas turbine engine is positioned. During gas turbine operation, including the start up procedure thereof, the air flow entering the combustor and mixing with the fuel is driven by the engine compressor, and thereby the air flow rate is a function of the compressor speed or the engine speed. Thus, the fuel/air ratio range for light-off is primarily determined by a light-off zone of a fuel flow rate and an engine speed rate. The light-off zone is affected by the atmospheric temperature and the altitude where the engine is positioned. This will be further discussed hereinafter. Therefore, it is usually difficult to achieve a quick light-off under various temperature and altitude conditions, particularly in cold weather unless the precise light-off zone can be identified under those conditions.
In contrast to the conventional manner of fuel injection for light-off, in which the fuel injection rate is generally represented by a linear or monotonic curve, the present invention provides a novel manner of injecting fuel into the combustor for achieving light-off in which fuel is injected at a varying rate which can be represented by a curve having at least one high frequency, for example, an oscillatory profile representing instant changes of the rate providing various instant fuel/air ratios in order to locate the light-off zone associated with the specific temperature and altitude conditions while reducing the quantity of fuel injected into the combustor. This will be further discussed with reference to the drawings and embodiments hereinafter.
In one embodiment of the present invention, a plurality of sensors are provided to measure the temperatures of the fuel and air flow to be injected into the combustor, the forward flight velocity ram quantity which is measured as the pressure difference between the inlet and outlet of the gas turbine engine, and the speed of the engine. The measured data is processed to determine a minimum engine speed at which the introduction of a predetermined first fuel flow level begins. The temperature of the exhaust gas flow is also measured to determine if light-off occurs. The light-off time is measured and stored in a database along with all other measurement data for reference in a future light-off of the gas turbine engine occurring under similar altitude and temperature conditions. Thus, the predetermined first fuel flow level, the criteria for the minimum engine speed for the introduction of the predetermined first fuel flow level, and the profile of the curve representing the varying fuel injection rate can be adjusted to achieve a shorter light-off time based on the information stored in the database.
The foundation of the present invention is based in part on the principle that xe2x80x9cat any given air mass flow, the range of air/fuel ratio within which the mixture can be ignited is smaller than that for which stable combustion is possible once ignition has occurredxe2x80x9d. This principle is stated in xe2x80x9cGas Turbine Theory by H. Cohen, G. F. C. Rogers and H. I. H. Saravanamuttooxe2x80x9d. The increasingly oscillatory light-off fuel flow will intersect the light-off zone and once the engine is lighted-off, the combustion will be sustained at a lower fuel flow because light-off of the fuel/air mixture occurs at a particular moment within a very small local area such that a light-off occurrence is primarily determined by the instant fuel/air ratio, or by the instant fuel flow rate. On the other hand, stable combustion is more effectively determined by the average fuel/air ratio or by the average fuel flow rate. The average fuel flow of every cycle of the light-off fuel flow is lower than the maximum fuel flow rate in the same cycle. Thus, quick light-off of a gas turbine engine can be achieved and continuous combustion can be sustained even in very cold weather, and excess fuel injection can thereby be avoided.
Other advantages and features of the present invention will be better understood with reference to preferred embodiments and drawings of the present invention described hereinafter.